How did Harlem come to be settled by so many blacks

Back in the day, African American people didn't have many opportunities in America, regarding jobs or housing. There weren't many places that would have them, simply because of their skin color. However, Harlem wasn't one of those places - many black people started moving there so as the escape the oppression. The more black people settled in, the more white people started abandoning Harlem, until it became mostly a black community. Harlem is also important culturally for African Americans, given that it is a symbol of their art and renaissance.